Impact of Distinct Ca$^{2+}$ Sources on the Physical Properties of Alumina-based Macroporous Refractories for Thermal Insulation at High Temperature

Abstract

Calcium aluminate cement (CAC), calcium carbonate (CaCO$_3$), calcium hydroxide [Ca(OH)$_2$] and calcium oxide (CaO) were investigated in alumina-based macroporous compositions for $\textit{in situ}$ formation of hibonite (also known as CaO.6Al$_2$O$_3$ or CA$_6$). Due to its volumetric expansion, this phase could counteract the linear shrinkage observed when macroporous ceramics are fired. In order to evaluate the impact of each Ca$^{2+}$ source on the physical properties, different formulations were processed and characterized for their total porosity, crushing strength and linear shrinkage. CaCO$_3$-containing samples presented high porosity and small dimensional changes after thermal treatment. Thus, a novel composition was formulated with $CaCO_3$ in order to result in 100 % CA$_6$ on thermodynamic equilibrium. Besides the usual properties, its $\textit{in situ}$ changes in the hot elastic modulus ($E_{\rm in\ situ}$) and thermal conductivity ($k_{\rm eff}$), were evaluated. This composition presented promising results, as a constant and low $k_{\rm eff}$, low linear expansion and Young modulus increase at low temperatures. These properties make it possible to produce more efficient insulators with better performance in service.